measure1Port

      measure1Port    makes a,b wave measurement at specified port
          rec = measure1Port(vnakit,settings,tx)
          input:
              vnakit: object (the board),
              settings: vnakit settings object,
              tx: (int) port number of transmitter (ie. ports['Tx1'])
          output:
             rec: [6,num_pts]
                  reciever recording with port tx as transmitter
         rec = measure1Port()
              measure 1 port without changing settings.